1. Field of the Invention
The present invention relates to an organic EL (electroluminescence) display or other image processing apparatus applying predetermined processing to an input image to display the image on a display unit and a method of the same.
2. Description of the Related Art
In an image display device, for example, a liquid crystal display, a large number of pixels are arranged in a matrix and the light intensity is controlled for every pixel in accordance with the image information to be displayed so as to display an image. This same is true for an organic EL display etc. An organic EL display is a so-called self light emitting type display having a light emitting element in each pixel circuit and has the advantages that the viewability of the image is higher in comparison with a liquid crystal display, a backlight is unnecessary, the response speed is high, etc. Further, it greatly differs from a liquid crystal display etc. in that the luminance of each light emitting element is controlled by the value of the current flowing through it. That is, each light emitting element is of the current controlled type.
An organic EL display, in the same way as a liquid crystal display, may be driven by a simple matrix and an active matrix system, but while the former has a simple structure, it has the problem that realization of a large sized and high definition display is difficult. For this reason, much effort is being devoted to development of the active matrix system of controlling the current flowing through the light emitting element inside each pixel circuit by an active element provided inside the pixel circuit, generally, a thin film transistor (TFT).
FIG. 1 is a circuit diagram of the configuration of first example of the configuration of an active matrix type organic EL display (see for example U.S. Pat. No. 5,684,365 and Japanese Unexamined Patent Publication (Kokai) No. 8-234683).
A pixel circuit 10 of FIG. 1 has a p-channel thin film field effect transistor (hereinafter referred to as a “TFT”) 11, an n-channel TFT 12, a capacitor C11, and a light emitting element 13 constituted by an organic EL element. Further, in FIG. 1, DTL indicates a data line, and WSL indicates a scanning line. An organic EL element has a rectification property in many cases, so sometimes is referred to as an organic light emitting diode (OLED). The symbol of a diode is used as the organic EL element in FIG. 1 and the other figures, but a rectification property is not always required for the organic EL element in the following explanation. In FIG. 1, a source of the TFT 11 is connected to a power supply potential VCC, and a cathode of the light emitting element 13 is connected to a ground potential GND. The operation of the pixel circuit 10 of FIG. 1 is as follows.
When the scanning line WSL is made a selected state (high level here) and a write potential Vdata is supplied to the data line DTL, the TFT 12 becomes conductive, the capacitor C11 is charged or discharged, and the gate potential of the TFT 11 becomes Vdata.
When the scanning line WSL is made a non-selected state (low level here), the data line DTL and the TFT 11 are electrically separated, but the gate potential of the TFT 11 is held stably by the capacitor C11.
The current flowing through the TFT 11 and the organic EL element 13 becomes a value in accordance with a gate-source voltage Vgs of the TFT 11, while the light emitting element 13 is continuously emitting light with a luminance in accordance with the current value. As in the above, the operation of selecting the scanning line WSL and transmitting the luminance information given to the data line to the inside of a pixel will be referred to as “writing” below. As explained above, in the pixel circuit 2a of FIG. 1, if once the Vdata is written, the light emitting EL element 13 continues to emit light with a constant luminance in the period up to the next rewrite operation.
FIG. 2 is a circuit diagram of a second example of the configuration of a pixel circuit in an active matrix type organic EL display.
A pixel circuit 20 of FIG. 2 has a p-channel TFT 21, a TFT 22, an n-channel TFT 23, a TFT 24, a capacitor C21, and a light emitting element 25 constituted by an organic EL element. Further, in FIG. 2, DTL indicates a data line, WSL indicates a scanning line, and ESL indicates an erasing line. An explanation will be given below of the operation of this pixel circuit 20 while referring to the timing chart shown in FIGS. 3A to 11E.
First, in a state (period) <1>, as shown in FIGS. 3C and 3D, a scanning signal WS applied to the scanning line WSL and an erasing signal ES applied to the erasing line ESL are set at the high level. Due to this, the TFT 24 and the TFT 23 become an ON state, the TFT 22 becomes an OFF state, and a charge in accordance with the data VDATA amount is stored in the capacitor C21 by the data line DTL.
In a state (period) <2>, as shown in FIGS. 3C and 3D, the scanning signal WS to the scanning line WSL and the erasing signal ES to the erasing line ESL are set at the low level. Due to this, the TFT 24 and the TFT 23 become the OFF state, the TFT 22 becomes the OFF state, and a current in accordance with the charge stored in the capacitor C21 flows in the EL light emitting element 25 through the TFT 21. This current is maintained until the signal ES applied to the erasing line ESL becomes the high level.
In a state (period) <3>, as shown in FIG. 3D, the erasing signal ES to the erasing line ESL is set at the high level. Due to this, the TFT 23 and the TFT 22 become the ON state, so the charge stored in the capacitor C21 is discharged through the TFT 23 and the TFT 22, and the light emission of the EL light emitting element 25 is turned OFF there.
In this way, the circuit of FIG. 2 controls the light emission period (DUTY) of the light emitting element 25 unambiguously by using one erasing line ESL by each pixel.
The fact that the light emitting element in an organic EL display has a characteristic deteriorating in proportion to the light emitting amount and time is generally known. Improvement of the characteristic of the light emitting element is hoped for. On the other hand, the display screen of the display is not always uniform, so deterioration of the light emitting elements in the screen is not uniform and becomes a factor of partial deterioration of the light emitting elements. In particular, in the display of the time etc., only that portion extremely deteriorates and drops in luminance. This is generally referred to as “burn-in” (hereinafter, partial pixel deterioration will be described as “burn-in”). Further, in a case where a plurality of light emitting elements are used or even in the case of a single light emitting element having a plurality of emission wavelength components, often the deterioration characteristics do not match. In this case, in the deteriorated pixel portion, the white balance becomes off and that portion appears colored.
To deal with burn-in of the screen due to deterioration of the display elements accompanying the light emission time, it has been considered best to improve the light emission lifetime of the display element material. Other than improving the material, in the past burn-in has been prevented by using circuits positively discharging the held capacitance of pixels (see for example Japanese Unexamined Patent Publication (Kokai) No. 2002-169509) to suppressing the unrequired light emission time. Further, an apparatus using a screen saver etc. to relieve burn-in has been proposed (see for example Japanese Unexamined Patent Publication (Kokai) No. 2002-207475).
However, while it is possible to improve the light emission lifetime of the display element material so as to extend the light emission lifetime of the display element material in a self light emitting type display somewhat, it is impossible to completely eliminate burn-in in principle. Further, looking at the video signal to be displayed on the display device, depending on its application, sometimes only a video signal easily causing burn-in is input. That is, burn-in cannot be prevented by only just improving the service life of the conventional material. Further, so long as the service life of the material is not extended, burn-in of the screen cannot be alleviated. It has therefore been necessary to rely on developments in this field such as the speed and costs of material development.
Circuits that positively discharge the held capacitance of the pixels disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-169509 and the circuits disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2002-207475) cannot however compensate for or ease the deterioration of the emission luminance accompanied with burn-in, that is, deterioration of the pixels, to an extent suitable for practical use.